Autocrine growth stimulation of thymocyte precursors in the thymus

A thymocyte specific growth peptide (TGP) has previously been purified from bovine thymus. In the present study TGP activity is demonstrated in the conditioned medium from a murine thymic leukemia cell line with prothymocyte phenotype, but is absent from media of cultured thymic epithelium. This murine TGP shows specificity for immature thymocytes and growth-stimulating kinetics identical to bovine TGP. Murine and bovine TGP show different chromatographic behaviour which may indicate biochemical differences. The results suggest that the immature thymocyte population includes both TGP-producing and TGP-responding cells. Our finding of TGP activity in fresh homogenates of normal mouse thymocytes supports this interpretation. We propose that TGP acts as an autocrine growth factor for rapidly cycling immature cortical thymocytes.